Wasp nest sprayed but still wasps flying in?

We live in Belgium where it is usual for the local firemen to deal with wasps nests - they came and sprayed the entrance of the nest last night- we have a large wasps nest in the cavity wall in my sons bedroom - the wasps are getting in/out through the roof outside. I can still hear buzzing tonight in the wall where the nest is and I've seen wasps still flying in and out. I assumed they would be killed outright - should I be concerned and call back the fire station or do they take a while to die off?